The purpose of this study is to assess the efficacy and safety of treatment with definitive chemoradiotherapy (dCRT) + pembrolizumab (MK-3475) compared to treatment with dCRT + placebo with respect to Event-free Survival (EFS) and Overall Survival (OS) in: * participants whose tumors express Programmed Death-Ligand 1 (PD-L1) Combined Positive Score (CPS) ≥10 * participants whose tumors express PD-L1 CPS ≥1 * all participants The primary study hypotheses are that dCRT+ pembrolizumab is better than dCRT + placebo with respect to: * EFS in participants whose tumors express PD-L1 CPS ≥10 * EFS in participants whose tumors express PD-L1 CPS ≥1 * EFS in all participants * OS in participants whose tumors express PD-L1 CPS ≥10 * OS in participants whose tumors express PD-L1 CPS ≥1 * OS in all participants